The cells of metal-air batteries utilize a negative electrode based on a metal such as zinc, iron, or lithium, coupled to an air electrode. The electrolyte most commonly used is an aqueous alkaline electrolyte.
During discharge of such a battery, oxygen is reduced at the positive electrode and the metal is oxidized at the negative electrode:
Discharge at the negative electrode: M→Mn++n e−
Discharge at the positive electrode: O2+2H2O+4e−→4OH−
The advantage of metal-air systems lies in the use of a positive electrode of infinite capacity, as the oxygen consumed at the positive electrode does not need to be stored in the electrode but can be collected from the ambient air. Metal-air type electrochemical generators are therefore known for their high specific energies, which can reach hundreds of Wh/kg.
Air electrodes are used in alkaline fuel cells for example, which are particularly advantageous over other systems because of the favorable reaction kinetics at the electrodes and because of the absence of noble metals such as platinum. Metal-air batteries are also used in hearing aids for example.
An air electrode is a porous solid structure, usually of carbon particles, in contact with the liquid electrolyte. The interface between the air electrode and the liquid electrolyte is a so-called “triple contact” interface where the active solid material of the electrode, the gaseous oxidant, that is to say the air, and the liquid electrolyte are present simultaneously. A description of different types of air electrodes for zinc-air batteries is described for example in the article by Neburchilov V. et al, entitled “A review on air cathodes for zinc-air fuel cells,” Journal of Power Sources 195 (2010) p. 1271-1291.
When a metal-air battery is to be recharged electrically, the direction of current is reversed. Oxygen is produced at the positive electrode and the metal is redeposited by reduction at the negative electrode:
Recharge at the negative electrode: Mn++n e−→M
Recharge at the positive electrode: 4 OH−→O2+2H2O+4 e−
Metal-air batteries work very well when discharging, but multiple problems remain unsolved when charging.
There are many disadvantages to using an air electrode in the recharging direction to carry out an oxidation reaction with oxygen release. The porous structure of the air electrode is fragile. It has been observed that this structure is mechanically destroyed by the release of gas when it is used to produce oxygen by oxidation of a liquid electrolyte. The hydraulic pressure generated in the electrode by the gas production is sufficient to break the binding between the carbon particles that compose the air electrode.
The degradation of the air electrode when it is used to charge or recharge the metal-air battery greatly reduces the life of the battery. This is one of the main causes of the limited commercial development of electrically rechargeable metal-air batteries.
One means of protecting the air electrode against degradation is to use a second positive electrode referred to as a second “charging” electrode, which is used for the oxygen release reaction.
The air electrode is then decoupled from the charging electrode and only the latter is used during the charging phase. The air electrode remains inert during the charging phase and retains a fixed potential. U.S. Pat. No. 3,532,548 of Z. Starchurski describes an example of a zinc-air battery with a second auxiliary electrode used for the charging phase.
Problems may also arise at the negative electrode during the recharging of a metal-air battery, for example a zinc-air battery.
When charging, the metal ions Mn+ are reduced at the negative electrode and are deposited in their metal form M as soon as the potential at this electrode is sufficiently negative. A uniform and homogeneous deposition of metal on the electrode is desired to ensure good performance during the charging and discharging cycles of this electrode.
However, it was found that under certain conditions, the metal could be deposited as a foam with poor adherence to the surface of the electrode and could thus detach from the electrode, causing a loss of active material and therefore a loss of battery capacity. In other cases, it was found that the metal could be deposited in the form of irregular growths called dendrites. These dendrites can grow to reach the positive electrode during charging, causing an internal short-circuit that prevents charging.
In an attempt to solve these problems and produce a homogeneous zinc deposit during the charging phase, patent WO 2014 083268 A1 proposes maintaining the potential of the negative electrode below a critical threshold. This potential is obtained by measuring the voltage between the air electrode, at a fixed potential during charging, and the negative electrode.
The above considerations relate to a single metal-air cell. However, batteries are usually composed of a plurality of cells connected to one another in series, in parallel, or in a combination of serial and parallel connections. The charging of such a battery makes use of a single charger supplying the battery with direct current. The charger is connected to both terminals of the battery, respectively corresponding to the negative electrode of a cell located at one end of the battery and the charging positive electrode of the cell located at the other end of the battery.
When charging a battery of cells connected in series, the total current flowing through the battery remains constant even if the cells are not individually in the same charge state. The cells may also be at different voltages. The release of oxygen at the anode of a metal-air cell results in large fluctuations in voltage between the two charging electrodes of the cell. Indeed, unlike the case of a closed battery in which the positive and negative electrodes can both be charged and are therefore both at stable potentials, the charging positive electrode of a metal-air cell is not in equilibrium with the active material, as there is a release of oxygen to the outside.
Large voltage fluctuations in a metal-air cell can intensify irregularities in the deposition of metal on the negative electrodes of the cells of such a battery.
Charge control devices for conventional two-electrode batteries allow distributing the charging current in the battery cells to ensure that all cells are fully charged. These charge control devices monitor the voltage at the terminals of a cell in order to identify where the cell lies on a previously obtained current-voltage charge curve. Charging is stopped when the voltage at the cell terminals exceeds a characteristic charging end point. These charge control devices are not suited for a battery of metal-air cells in which voltage fluctuations render inaccurate the charge state data of a cell, and where it is important to be careful not to exceed a threshold potential at the negative electrode as described above.
The above constraints make it very difficult to provide effective control of a battery composed of metal-air cells in order to increase the number of charging and discharging cycles of the battery, and thus advantageously give the battery a longer life. There is therefore a need for a charge control method for a battery composed of metal-air cells, and for a charge manager for such a battery.